FIG. 1 is a schematic representation of an example of a network infrastructure, such as a wireless communication network or wireless communication system, including a plurality of base stations eNB1 to eNBs, each serving a specific area surrounding the base station schematically represented by the respective cells 1001 to 1005. The base stations are provided to serve users within a cell. A user may be a stationary device or a mobile device. Further, the wireless communication system may be accessed by IoT devices which connect to a base station or to a user. IoT devices may include physical devices, vehicles, buildings and other items having embedded therein electronics, software, sensors, actuators, or the like as well as network connectivity that enable these devices to collect and exchange data across an existing network infrastructure. FIG. 1 shows an exemplary view of only five cells, however, the wireless communication system may include more such cells. FIG. 1 shows two users UE1 and UE2, also referred to as user equipment (UE), that are in cell 1002 and that are served by base station eNB2. Another user UE3 is shown in cell 1004 which is served by base station eNB4. The arrows 1021, 1022 and 1023 schematically represent uplink/downlink connections for transmitting data from a user UE1, UE2 and UE3 to the base stations eNB2, eNB4 or for transmitting data from the base stations eNB2, eNB4 to the users UE1, UE2, UE3. Further, FIG. 1 shows two IoT devices 1041 and 1042 in cell 1004, which may be stationary or mobile devices. The IoT device 1041 accesses the wireless communication system via the base station eNB4 to receive and transmit data as schematically represented by arrow 1061. The IoT device 1042 accesses the wireless communication system via the user UE3 as is schematically represented by arrow 1062.
The wireless communication system may be any single-tone or multicarrier system based on frequency-division multiplexing, like the orthogonal frequency-division multiplexing (OFDM) system, the orthogonal frequency-division multiple access (OFDMA) system defined by the LTE standard, or any other IFFT-based signal with or without CP, e.g. DFT-s-OFDM. Other waveforms, like non-orthogonal waveforms for multiple access, e.g. filter-bank multicarrier (FBMC), may be used.
In a wireless communication network, like the one depicted in FIG. 1, it may be desired to locate a UE with a certain accuracy in a cell. One approach to locate a UE within a cell is based on an observed time difference of arrival (OTDOA) estimation that may be used in cellular communication networks, such as LTE, and which is a downlink positioning method that relies on the calculation of time of arrival (ToA) estimates using position reference signals (PRS) receives at the user equipment UE from one or more surrounding base stations (eNB), as is described, for example, in references [2] and [3]. PRS sequences are downlink signals that are designed for positioning purposes and that are broadcast to all radio terminals within a cell. The PRS sequences are radiated with the same transmit power from the antenna of the base station or the remote radio head (RRH) in all directions to cover all users at any location of the cell, i.e., to provide a cell-wide coverage. To distinguish the PRS sequences from different cells, each PRS sequence has associated therewith a cell-specific identifier also referred to as a physical cell identifier (PCI). The PCI is unique in a specific area and is used to identify the cell and thus the PRS sequence. At least three timing measurements from geometrically dispersed base stations may be used, relative to the UE's internal time base, in order to obtain a unique position in a plane. Four base stations are needed to obtain a unique position in a three-dimensional space as is described in reference [4].
FIG. 2 is a schematic representation of a OTDOA measurement using three base stations, wherein the figure is based on an image taken from reference [4]. The base stations eNB1 to eNB3 send out respective PRS sequences having associated therewith a PCI. Base station eNB1 sends out the PRS sequence PRS1, base station eNB2 sends out the PRS sequence PRS2, and base station eNB3 sends out the PRS sequence PRS3. The base stations eNB1 to eNB3 serve different cells of the wireless communication network. Although FIG. 2 shows a transmission of the respective PRS sequences in only one direction from each base station, as explained above, each base station transmits the sequences in all directions to cover all users at any location within the respective cell. In the example of FIG. 2, a user equipment is assumed to be at location 108. The UE at location 108 receives from the respective base stations the PRS sequences PRS1 to PRS3.
The UE at location 108 measures three ToAs τ1, τ2, τ3 relative to the UE internal time base. The base station eNB1 is selected as a reference base station, and two OTDOAs are obtained by subtracting the ToA of the reference base station eNB1 from the ToA measurements of the other base stations yielding as observed time differences of arrival the values t2,1=τ2−τ1 and t3,1=τ3−τ1, also referred to as relative signal timing differences (RSTDs). The relative signal timing differences are fed back to the base station serving the user equipment at location 108, as well as to a location server. The location server may be part of the base station or it may be an element separate from the base station, as is indicated at 110 in FIG. 1. The location server 110 may be part of the overall network structure and may be connected to each of the base stations shown in FIG. 1, although only one connection is shown in dotted lines in FIG. 1. The RSTD values are related to the geometric distances between the UE and the base stations and define hyperbolas around the respective locations of the base stations, as is indicated by line τ3−τ1 and line τ2−τ1 in FIG. 2. Based on the knowledge of the base station coordinates and the time offset between the UE and the reference base station eNB1, the location server may determine the position of the UE. In FIG. 2, since each ToA measurement τi has a uncertain accuracy, the hyperbolas are shown with a width illustrating the measurement uncertainty. The estimated UE location is the intersection area of the two hyperbolas.
For the ToA measurements by the UE and the RSTD reporting, the first arriving signal path from each base station has to be accurately estimated. In pure line-of-side (LoS) channel conditions, the ToA estimates reflect the first detected LoS peaks in a cross-correlation of the received signal with a PRS sequence, which is known at the UE, so that the ToA directly corresponds to the distances between the UE and the base stations. This allows accurate position estimates for the UE. In multipath channel environments, however, the ToA estimates, and therefore also the RSTD measurements, may be biased by an obstruction of the LoS path or by the non-line-of-side (NLoS) signal path components of the channel and, in such a situation, the UE may not correctly detect the first arriving signal path which may lead to erroneous distance information.
The above described approach for localizing a UE within the cell uses PRS sequences that are transmitted by a plurality of base stations. Each base station sends out the same PRS sequence in all directions to cover all users. To allow for a localization at least three base stations are needed. Further, the radio propagation channel may suffer from a multipath propagation and shadowing or fading conditions so that the RSTD measurements may not be accurate. Multipath is a phenomenon that happens in the channel of mobile systems when the transmitted signal arrives at the receiver via different paths due to reflection, diffraction and scattering resulting in fading. This is schematically represented in FIG. 3 representing a UE within a cell i that is served by the base station BS transmitting the same PRS sequence with the associated cell identifier in all directions, namely the sequence PRSi. The PRS sequence PRSi may not be directly received at the UE due to an obstacle 1121 scattering or shadowing the signal so that there is a path loss behind the obstacle 1121. The UE may receive the sequence PRSi due to a reflection of the signal at the obstacle 1122 and/or due to a diffraction at the obstacle 1123. In other words, there is only one transmitted signal PRSi, however, the obstacles 1121 to 1123, like buildings, hills and trees, in the signal paths cause the signal to arrive at the UE from various directions with different delays. The multipath may be a source of error in the ToA estimation, for example, when there is no line of side path, even if the receiver UE detects the first arriving path.
To improve the position accuracy in multipath channel scenarios, a number of NLoS error mitigation techniques have been described for time-based location estimation, see for example references [6], [7], [8], [9], [10] and [11]. One NLoS error mitigation technique may assume that the NLoS corrupted ToA measurements are only a small portion of the total number of measurements, i.e., some of the links between the UE and the base station contain an LoS channel path. Another approach may detect NLoS corrupted ToA measurements due to their inconsistency with the expected measurement for a LoS scenario so that NLoS links between the UE and the base station may be identified and ignored for the localizing of the UE position as is, for example, described in references [8] and [12]. Yet other approaches may use all links between the UE and the base station and may introduce a weighting or scaling of ToA measurements to minimize the NLoS contributions, as is described in reference [13], or detect the NLoS errors and use the information to calculate all possible UE locations, as is described in reference [14].
As is shown in FIG. 1, the base stations of the wireless communication network include a plurality of antennas ANT, for example formed by an antenna array including a plurality of antenna elements, and the UE may also include more than one antenna. In scenarios in which both the UE and the base station are equipped with a plurality of antennas, location-independent parameters may be exploited in addition to the OTDOA measurements of the LoS or NLoS path components, for example an angle of arrival (AoA) at the UE and an angle of departure (AoD) at the base station may be used. Instead of detecting only NLoS errors and removing the influence of these errors, examples of localization techniques may benefit from the NLoS channel propagation by exploiting the geometrical relationship of possible UE locations implied by the NLoS path components. Such techniques are described, for example, in references [15] and [16] and rely on a parametric description of the multipath channel propagation environment, assuming knowledge of the path-dependent parameters AoD, AoA and the path distance d.
To improve position accuracy in multipath channel scenarios, it has been proposed in reference [17] to extend the ToA measurement report sent by the UE to the serving base station by multiple RSTD measurements. Instead of sending only a single RSTD measurement report per UE-cell link, which corresponds to what the UE considers as the LOS peak in the cross-correlation function, more than one RSTD value is fed back to the base station. The multiple RSTD values, for example, correspond to multiple peaks of the PRS correlation function that represents the ToA estimates of the multipath components. The location server may perform the UE localization based on multiple RSTD hypotheses that may result in an improved positioning performance. This approach may also be applied in an interference scenario, e.g., in a situation in which an interference occurs due to strong correlations of PRS sequences sent from surrounding base stations. The multiple RSTD measurements in such a scenario may correspond to the peaks related to interfering signals. This may also be handled by multiple RSTD hypotheses at the location server. An example illustrating the multiple RSTD measurement approach is shown in FIG. 4, where for eNB1 and eNB3 there are two ToA estimates corresponding to the LOS path components of the channels, whereas for eNB2 there are two ToA estimates which are caused by a weak LOS component or an interfering signal, In this example, the RSTD values reported to the location server are given by (τ1-τ2; τ1-τ3; τ1-τ2,1; τ1-τ2,2) (see reference [17]). The multiple RSTD values for eNB2 are used to calculate multiple hypothesis of the UE position at the location server (see hatched area in FIG. 4). Considering the likelihood function of possible UE locations, it can be observed that the probability to estimate a more accurate UE position increases significantly with a multiple RSTD UE feedback compared to a single RSTD UE feedback (see reference [17]).
As mentioned above, the UE may be equipped with a plurality of antennas, for example, an antenna array, for transmitting or receiving radio signals. Each antenna has an antenna pattern describing a response of the antenna to a signal received at the antenna from a certain direction or angle. For example, the antenna pattern indicates that signals received at the antenna from a first direction are amplified, while signals received from a second direction are damped or suppressed. The respective antennas of the UE may have the same or different antenna patterns. FIGS. 5A-D shows examples for antenna patterns of different antennas of a UE and how LoS and NLoS path components in a multipath channel environment may be received at such an antenna. FIG. 5A shows an antenna pattern for a first antenna ANT1 of the UE. The antenna ANT1 has an antenna pattern including a main lobe 1141, and respective side lobes and nulls 118a1, 118b1 and 118c1. Signals received in a direction in which the main lobe 1141 are directed are received with an increased sensitivity, whereas signals received from a direction into which the respective side lobes and nulls 118a1, 118b1 118c1 are directed are received with a reduced sensitivity. For example the signals received in the directions of the main lobe may be amplified, while those signals received from the direction of the side lobes may be damped or suppressed. FIG. 5B shows an antenna pattern for a second antenna ANT2 of the UE. The second antenna ANT2 has an antenna pattern or directivity including a main lobe 1142 and respective side lobes and nulls 118a2, 118b2 and 118c2. In the example of FIG. 5B the antenna pattern of the second antenna ANT2 is the same as the antenna pattern of the first antenna ANT1 shown in FIG. 5A. In the examples of FIG. 5A and FIG. 5B, the main lobes are oriented along the x-axis of the x/y plane at an angle of 0°. The side lobes are directed in directions between 0° to 360°. In accordance with other examples, the antenna pattern of the second antenna ANT2 may be different from the antenna pattern of the first antenna ANT1. FIG. 5C shows an example for such a different antenna pattern, which may be the antenna pattern for the second antenna ANT2, or as is shown in this example is an antenna pattern for a third antenna ANT3 of the UE. The antenna pattern or directivity of the third antenna ANT3 also has a main lobe 1143, and respective side lobes and nulls 118a3, 118b3 and 118c3. In the example of FIG. 5C, the main lobe is oriented at an angle with respect to the x-axis of the x/y plane, and the side lobes are directed in different directions. When compared to the first and second antenna patterns, the respective lobes of the third antenna patterns are directed into different directions so that a signal received from a first direction, into which the main lobes 1141 and 1142 of first and second antennas are directed, receives a higher amplification by the first and second antennas than by the third antenna, or, in other words, is damped by the third antenna.
FIG. 5D shows how LoS and NLoS path components in a multipath channel environment may be received at an antenna of the UE, like the one shown in FIG. 5A. A base station BS transmits a signal carrying a PRS sequence PRSi, and there is a direct path, the LOS path, between the base station BS and the UE as well as indirect paths, the NLOS paths, along which the signal PRSi is reflected at objects e.g. 112. The signal received via the LOS path is received from a direction to which one of the upper side lobes or null 118a1 of the antenna ANT1 are directed, whereas the indirect or NLOS path component is received at the main lobe 1141. The signal on the LOS path experiences a damping or even a nulling by the antenna ANT1 so that the UE may not correctly detect the first arriving signal path at the antenna ANT1, which would be the LOS path, due the impact of the antenna pattern of the antenna ANT1 which may suppress the signal strength of the LOS.